1. Field of the Invention
The present invention relates to differential phase-shift keying (DPSK) in telecommunication, and more specifically, it relates to methods in DPSK for converting a phase-keyed signal to an intensity-keyed signal.
2. Description of Related Art
Phase-shift keying (PSK) is a digital modulation scheme that conveys data by changing, or modulating, the phase of a reference signal (the carrier wave). Any digital modulation scheme uses a finite number of distinct signals to represent digital data. In the case of PSK, a finite number of phases are used. Each of these phases is assigned a unique pattern of binary bits. Usually, each phase encodes an equal number of bits. Each pattern of bits forms the symbol that is represented by the particular phase. The demodulator, which is designed specifically for the symbol-set used by the modulator, determines the phase of the received signal and maps it back to the symbol it represents, thus recovering the original data. This requires the receiver to be able to compare the phase of the received signal to a reference signal—such a system is termed coherent.
Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme).
In telecommunication technology, differential phase-shift keying (DPSK) requires a decoding method in order to convert the phase-keyed signal to an intensity-keyed signal at the receiving end. The decoding method can be achieved by comparing the phase of two sequential bits. In principle, it splits the input signal beam into two channels with a small delay before recombining them. After the recombination, the beams from the two channels interfere constructively or destructively. The interference intensity is measured and becomes the intensity-keyed signal. To achieve this, one channel has an optical path longer than the other one by a distance equivalent to the photon flight time of one bit. For instance, in a 40 Gbit per second system, one bit is equal to 25 ps, and light travels 7.5 mm in that period. In this example, the optical path difference (OPD) between the two channels is 7.5 mm.
The Mach-Zehnder type interferometer with a desired OPD between the two channels is currently used for decoding purposes. Because of the properties of optical interference, a change in OPD can greatly affect interference intensity. Moreover, the optical path in each arm is much longer than its difference. Therefore, a sophisticated temperature control is required to maintain the optical path in each arm in order to assure that the change in the OPD is much less than a small fraction of one wavelength, e.g., ˜10 nm. This is difficult and expensive, especially for an interferometer with a long optical path.